The recent growth in many areas of biotechnology has increased the demand to perform a variety of studies, commonly referred to as assays, of biochemical systems. These assays include, for example, biochemical reaction kinetics, DNA melting point determinations, DNA spectral shifts, DNA and protein concentration measurements, excitation/emission of fluorescent probes, enzyme activities, enzyme-cofactor assays, homogeneous assays, drug metabolite assays, drug concentration assays, dispensing confirmation, volume confirmation, solvent concentration confirmation and solvation confirmation. Since most components of biochemical systems absorb radiation in the ultraviolet (UV) region of the electromagnetic spectrum (200 nm to 400 nm), UV absorption spectroscopy may be used to study these systems. In addition, UV absorption spectroscopy offers the advantages of relatively high precision and accuracy.
Assays of biochemical systems are carried out on a large scale in both industry and academia, so it is desirable to have an apparatus that allows these assays to be performed in a convenient and inexpensive fashion. Because they are relatively easy to handle and low in cost, microplates are often used for such studies. Microplates typically consist of a plurality of individual wells formed of polymeric materials. Each well includes sidewalls and a bottom so that an aliquot of a sample may be placed within each well. The wells may be arranged in relatively close proximity in a matrix pattern, allowing samples to be studied individually or as a group. Common sizes for microplates include matrices having dimensions of 4.times.6 (24 wells) or 8.times.12 (96 wells), although larger microplates are also used that may include matrices of hundreds or even thousands of wells.
Typically, the materials used to construct a microplate are selected based on the samples to be assayed and the analytical techniques to be used. For example, the materials of which the microplate is made should be chemically inert to the components of the sample, and the materials should be impervious to radiation or heating conditions to which the microplate is exposed during the course of an experiment. Thus, a microplate used in assaying samples by UV absorption should have a UV permeable bottom sheet so that a substantial amount UV radiation can pass through each well and interact with the sample without being absorbed by the well bottom.
Despite the potential advantages of employing microplates having UV permeable bottom sheets, there has been limited progress in manufacturing such microplates. One problem in designing these microplates relates to the polymeric materials that are typically used for microplate construction. In particular, these polymeric materials usually have relatively high UV absorption probabilities. Absorption of UV radiation by the polymeric materials results in the chemical and physical degradation of the microplates. Therefore, to prolong the lifetime of these microplates, UV stabilizers specifically designed to absorb UV radiation are often added to the polymeric materials. As a result, most known microplates have exceptionally high UV absorption probabilities, rendering them useless for experiments in which UV absorption of samples is used.
U.S. Pat. No. 5,487,872 to Hafeman et al. (Hafeman) discloses a microplate designed for assaying samples with UV absorption techniques. Hafeman discloses a variety of materials from which the bottom surface of the microplate wells may be formed, including TPX.RTM. 4-methylpentene-1 polymer as the preferred material (Mitsui Petrochemical Industries, Japan). However, it is believed that microplates using this material for the well bottoms may have limited sensitivity in certain biochemcial experiments. For example, in nucleic acid studies, UV absorption in a range between approximately 260 nm to approximately 280 nm is studied, but TPX.RTM. has a relatively high optical density in this wavelength range.
Microplates having a quartz bottom plate glued to a molded body have also been produced. However, the cost of these microplates is often more than two orders of magnitude higher than the cost of a microplate formed entirely from polymeric materials, precluding their use for most studies. In addition, the materials used to bond the quartz bottom plate to the microplate body may leach into samples contained within the wells of the microplate, contaminating the samples and compromising the reliability of the experimental results. Furthermore, over time, the strength of the bond between the bottom plate and the body may deteriorate and form leaks between sample wells.
Hence, it remains a challenge in the art to provide a microplate that is relatively inexpensive, comparatively durable and includes well bottoms having an acceptable optical density across the entire useful range of the UV spectrum.